The concept of terahertz (THz) sources based on intra-cavity difference-frequency generation (DFG) in dual-wavelength mid-infrared (IR) quantum cascade lasers (QCLs) was proposed and experimentally demonstrated by Belkin et al. (See M. A. Belkin, et al., Nature Photon. 1, 288-292 (2007).) Room-temperature operation of these devices (called THz DFG-QCLs) followed. (See M. A. Belkin, et al., Appl. Phys. Lett. 92, 201101 (2008).) The active region in THz DFG-QCLs is quantum-engineered so as to produce both broadband mid-IR gain and giant intersubband nonlinearity for THz DFG. Upon application of a bias voltage, lasing commences at two mid-IR frequencies ω1 and ω2 and the THz output at ωTHz=ω1−ω2 is produced by DFG between the mid-IR beams in the same monolithic laser cavity. From a user perspective, these devices are similar in their production costs and operating simplicity to mid-IR QCLs.
The key figure of merit for this technology is the mid-IR-to-THz conversion efficiency η, which is defined as the ratio of the generated THz power to the product of the two mid-IR pump powers. Initial room-temperature devices demonstrated η≈5 μW/W2, limited by inevitable THz free-carrier absorption in the doped parts of the mid-IR QCL core region. (See M. A. Belkin, et al., Appl. Phys. Lett. 92, 201101 (2008).) To overcome this problem, a new type of THz DFG-QCL was developed, based on Cherenkov phase-matching. (See K. Vijayraghavan, et al., Appl. Phys. Lett. 100, 251104 (2012).) These devices are grown on semi-insulating InP substrates that have relatively low THz loss and high refractive index at THz frequencies (nTHz≈3.6), significantly higher than the group index of the mid-IR modes in the laser waveguide (ng≈3.35). In this situation, THz radiation produced by DFG leaks from the laser waveguide into the low-loss InP substrate at a Cherenkov angle (θTHz=cos−1(ng/nTHz)), and can thus be extracted along the whole length of the QCL waveguide. As a result, a dramatic improvement in mid-IR-to-THz conversion efficiency has been achieved in Cherenkov THz DFG-QCLs, with the value of η exceeding 0.6 mW/W2 for a ˜2-mm-long ridge waveguide QCL operating in 3.5-4 THz range (K. Vijayraghavan, et al., Nature Comm. 4, 2021 (2013)) and being approximately 10 times smaller for similarly-sized devices operating around 1-2 THz (K. Vijayraghavan, et al., Nature Comm. 4, 2021 (2013) and Y. Jiang, et al., J. Opt. 16, 094002 (2014), (Invited paper)). This value of η is obtained before correcting for an estimated 50% of THz collection efficiency of the THz power measurement setup. With this correction, the value of η exceeds 1 mW/W2 at 3.5-4 THz and drops to ˜0.1 mW/W2 for 1-2 THz operation. It has also been shown that Cherenkov DFG-QCL chips can provide continuously tunable THz emission spanning 1.2-5.9 THz range. (See Y. Jiang, et al., J. Opt. 16, 094002 (2014), (Invited paper)) To date the highest average powers achieved at room-temperature by Cherenkov THz DFG-QCLs have been varying from about 15-30 μW at 3.5-4 THz (K. Vijayraghavan, et al., Appl. Phys. Lett. 100, 251104 (2012) and Q. Y. Lu, et al., Appl. Phys. Lett. 104, 221105 (2014)) to 0.1-1 μW at ˜1-1.5 THz (Y. Jiang, et al., J. Opt. 16, 094002 (2014), (Invited paper) and Q. Y. Lu, et al., Appl. Phys. Lett. 101, 251121 (2012).)